The invention relates to a tensioner for a power transmission belt.
The Applicant has already proposed, in FR-A-No. 2 597 565, a tensioning device for a power transmission belt comprising a return spring associated with the shaft of a tensioning roller with which the belt cooperates for controlling the displacement of said roller under the action of said spring which is coated with elastomer forming a sheath defining at least one variable volume chamber filled with a liquid, said chamber being closed at one of its ends by a valve device through which the liquid flows into or out of said chamber and which introduces an effect of damping the vibrations likely to arise in the belt.
Such a device, which ensures tensioning of the belt--a condition sine qua non for satisfactory operation of the power transmission system--further damps the vibrations generated in the belt, particularly those due to the cyclic irregularities of the drive system or to the shocks caused by starting up and/or stopping of the apparatus coupled to the driven shaft which the belt drives. The results obtained by means of such a tensioner are satisfactory but the valve device introduces an operating dissymmetry which causes a very great difference between the maximum dynamic pressures of the liquid at the inlet and outlet of said chamber. Thus, in the case of a tensioner with two chambers defined by the elastomer sheath and for a transmission belt driving a plurality of driven shafts from the crankshaft of a motor vehicle, truck or similar engine, the ratio of the maximum dynamic pressures is at the minimum about 10, if not more, the highest pressure being of the order of 40 bars, for example. The result is severe constructional requirements for the device which must withstand such pressures, not only in so far as the elastomer is concerned but also the spring used.
With this situation in mind, it is an object of the invention to provide a tensioner for a power transmission belt which, while delivering the same results as those of the known device, is nevertheless simpler to construct.
It is also an object of the invention to provide such a tensioner which, in addition, exhibits improved characteristics with respect to the known device, not only in so far as its endurance and resistance are concerned, but also the possibility of operating over a wide temperature range.
It is a further object of the invention to provide such a tensioner which, while being shorter in length than the known device, has however the same characteristics, thus providing a considerable advantage for use thereof in all fields of application where space is limited, for example in the automobile industry where the dimensions of the accessories under the engine bonnet must be as small as possible.